Inhibiting popcorn polymer formation with tertiary amino naphthol compound



States Patent U.S. Cl. 260666.5 Claims ABSTRACT OF THE DISCLOSUREPopcorn polymer formation in processes for preparing synthetic rubber isinhibited by contacting the monomers with a tertiary amino naphtholcompound of the structure CHZNWRDD where R and R are alkyl radicals,fl-hydroxyalkyl radi cals, and oxydiethylene when R and R together forma single radical; and n is 1 or 2.

This invention relates to the inhibition of the undesirablepolymerization of olefinic monomers which gives rise to popcorn polymersand is particularly concerned with the use of certain tertiary aminonaphthol compounds and derivatives as popcorn polymer inhibitors inprocesses relating to the preparation of synthetic rubber.

In the preparation of synthetic rubber from such intermediates asstyrene and butadiene (e.g. SBR elastomers) undesirable spontaneouspolymerization often occurs in the recovery systems for the olefinicallyunsaturated monomers. Polymerization may occur to form either a clear,solid, aromatic solvent soluble polymer or to form an entirely differentcross-linked insoluble polymer, known, because of its appearance, aspopcorn polymer. While both types of this self-polymerization areobjectionable, the popcorn polymer, which is predominantly formed, isparticularly undesirable because it is self-propagating in the presenceof the monomer vapor or liquid and not only rapidly fouls the equipment,but is very diflicult to remove and control. When such popcorn polymersdo form it frequently becomes necessary to disassemble the equipment andmechanically remove the accumulations of unwanted polymer.

Much work has been done to find suitable inhibitors to prevent popcornpolymer formation. Nitrites and nitroso compounds have been found to beeffective as have N0 N 0 certain phenolic compounds, sulfur and somearomatic amines. However, most of these agents leave something to bedesired for commercially effective use. Some of the agents are difiicultto handle; others introduce colored impurities into the recoveredolefins; some, although having the desired properties, are too expensiveto be of commercial utility. One of the major objections to most of thepreviously used popcorn polymer inhibitors is their ineffectiveness inthe presence of seed; that is, where clean equipment is used, the priorart inhibitors will reduce the tendency for popcorn polymer to form, butonce a popcorn polymer seed is formed the inhibitors lose effectiveness.As will be seen, this invention provides popcorn ploymer inhibitorswhich retain their inhibiting properties even when popcorn polymer seedis in the system.

Because of the uniqueness of the popcorn polymer and the manner in whichit isformed, there is no correlation between popcorn polymer inhibitionand monomer stabiliz- 3,524,894 Patented Aug. 18, 1970 ers useful toprevent the premature polymerization of olefins during Shipping andstorage. Accordingly, many agents reported to be stabilizers forolefinic monomers are not as eflective as popcorn polymer inhibitors.Thus, the problem of popcorn polymer formation in the manufacture ofsynthetic rubber is peculiar to the monomer recovery system where themonomers are recovered by distillation.

The conventional closed system for the emulsion polymerrzation ofbutadiene with styrene (a representative synthet c rubber) comprises aconventional reaction vessel equipped with a stirring mechanism andnecessary heating or cooling means in which the monomers are caused topolymerize. After a suitable degree of polymerization is achieved, thepolymerization reaction is stopped by the addition of a suitable shortstopping agent. The resulting polymer latex is then allowed to flow intoa flash tank which is at or slightly above atmospheric pressure and atwhich time most of the residual butadiene is removed from the latex. Thegaseous butadiene is then removed from the flash tank and liquified forreuse. The butadiene degassed latex is then allowed to flow into aconventional vacuum flash tank where further butadiene and other dissolved gaseous materials are removed. The vacuum flash tank ismaintained at a temperature of bout F. It is in this vacuum flash tankthat the most ideal conditions for popcorn polymer formation existbecause the tank is at the proper temperature; the atmosphere above thelevel of latex contained in the tank contains about 2% or less ofbutadiene; and a certain amount of catalyst has vaporized and collectedon the inner exposed metal surfaces of the tank above the level of thelatex. These conditions will initiate popcorn polymer. The popcornpolymer will continue to grow as long as it is fed by a new supply oflatex containing a small proportion of butadiene and other polymerizablemonomer, such as styrene. The pipe lines leading to and from this vacuumflash tank are also ideal areas for popcorn polymer formation.

The latex is then pumped from the vacuum flash tank to a conventionalstyrene stripping column where the latex is passed counter-current to arising stream of steam causing the styrene to be removed from the latexwhere it is then recirculated in a conventional manner to the reactionvessel for polymerization with butadiene. In the styrene strippingcolumn popcorn polymer formation also tends to develop unless someprecautions are taken to prevent its development.

Preferably, the inhibitor is added to a flash tank used in the process.However, the inhibitor may be introduced to the monomer at any stage inthe manufacture of synthetic rubber, as for example during themanufacture, handling, storage, etc. of the intermediates. For example,the inhibitor can be introduced as the gaseous monomer is being passedthrough pipes, it can be mixed with the monomer in process tanks, or, asindicated, it can be introduced during the fractional distillation ofmaterials in the recovery systems of the rubber manufacturing process.In the preferred technique it is considered best to feed the monomerinto a flash tank or fractional distillation column of conventionaltype. The monomer is subject to fractional distillation usingconventional reboiling at the bottom of the column and withdrawal ofoverhead material at the top, condensing the overhead material andreturning a portion of it to the top of the column as reflux. Theinhibiting solution is continuously fed, preferably by spraying itssolution in water or monomer into the upper portion of the columnthrough which it descends. In other techniques the inhibitor can beintroduced to one or more of the monomers in any phase wherein themonomer is being circulated in the process.

The concentration at which the inhibitor is used will usually range from0.001 to about 5.0 percent by weight of the total monomers (i.e. aboutto 50,000 parts per million parts of monomer). At concentrations belowthis value the inhibiting effects are too small to be of significantvalue. On the other hand, greater amounts may be used, say up to butsuch large amounts are not required and are simply wasteful ofinhibitor.

The tertiary amino naphthol compounds and derivatives which are used toeffectively inhibit popcorn polymer formation according to thisinvention have the formula wherein R and R are selected from the groupconsisting of alkyl radicals having one to five carbon atoms,fi-hydroxyalkyl radicals having one to five carbon atoms, andoxydiethylene when R and R together form a single radical; and n is 1 or2.

Representative of the preferred compounds useful in this invention are:

1-dimethylaminomethyl-2-naphthol 1-diethylaminornethyl-Z-naphthol1-di-n-butylaminomethyl-2-naphthol l-di (fl-hydroxyethylaminomethyl)-2-naphthol 1-N-ethyl-N-,B-hydroxyethylaminomethyl-2-naphthol 4-(dimethylaminomethyl) -l-naphthol 2,4-bis(dimethylaminomethyl)-1-napl1thol 1-morpholinomethyl-Z-naphthol 2-morpholinomethyl-1-naphthol2,4-di (morpholinomethyl) -1-naphthol2-di-fi-hydroxyethylaminomethyll-naphthol 1-di(,8-hydroxypropyl)aminomethyl-Z-naphthol 2-di( fi-hydroxypropyl aminomethyll-naphthol2,4-bis(di-B-hydroxyethylaminomethyl)-1-naphthol2-dibutylaminomethyll-naphthol 2,4-bis(dibutylaminomethyl) -1-naphthol 1diamylaminomethyl-Z-naphthol Also useful are the equivalentwater-soluble salts of the aforedescribed compounds such as the oxalate,acetate, maleate, citrate, glycolate salts, and the like. The salts aresometimes advantageous because of their ready solubility in aqueouslatex media.

The tertiary amino naphthol compounds used in this invention are readilyobtained by the well-known Mannich reaction involving the reaction ofnaphthol starting material with a secondary amine and formaldehyde. Suchmethods of preparing the compounds are described by J. H. Buckhalter etal., I. Am. Chem. Soc. 68, p. 1894 (1946); H. R. Snyder et al., I. Am.Chem. Soc. 70, p. 4230 (1948); and R. L. Shriner et al., J. Am. Chem.Soc. 68, p. 946 (1946).

It is to be understood that the specific tertiary amino compoundsembodied herein can be used generally to prevent popcorn polymers in thepreparation of polymers and copolymers such as those obtained fromethylenically unsaturated monomers. For homopolymers, the unsaturatedmonomer will be a conjugated diolefin. The useful conjugated diolefinsare exemplified by butadienes such as butadiene-1,3, isoprene,cyanobutadiene-1,3, chloroprene, 2-phenylbutadiene,2,3-dimethylbutadiene-1,3, and the like. The copolymerizable monomerused in copolymer formation and which will normally comprise up to about70% of the mixture will be a mono-olefin containing a single CH =CHgroup having at least one of the free valence bonds attached to anelectronegative group. Such olefins include aromatic olefins such asstyrene, vinylnaphthalene, a-methylstyrene, p-chlorostyrene, etc.; thecarboxy containing monomers and the corresponding esters, nitriles, andamides such as acrylic acid, methacrylic acid, methylmethacrylate,acrylonitrile, methacrylamide, and the like. Preferably, this inventionwill be used in recovering the monomers used to make any butadiene-basedpolymer latex.

The following illustrative examples are set forth for evaluation of theunusual effectiveness of the compounds of this invention as popcornpolymer inhibitors. The technique used for measuring inhibitingeffectiveness was as follows:

A one-half gram sample of popcorn polymer seed from a styrene-butadienerubber (SBR) plant flash tank (formed from styrene containing a smallamount of butadiene) was placed in a seven ounce bottle previouslyflushed with nitrogen and containing 30 ml. of freshly distilledstyrene. The bottle was capped with a self-sealing cap and 1 ml. ofliquid butadiene and the inhibitor was added through the cap with ahypodermic syringe.

The inhibitor concentration was 0.1% by Weight of the monomers. Thebottles were kept at'the elevated temperature of F. and observed untilpopping (i.e. polymerization and formation of popcorn polymer) wasfound. The number of days required for this condition to occur wasrecorded. The data are reported in Table I which follows.

The foregoing results show that compounds embodied in my invention giveoutstanding results, which are in contrast to the poor performance ofvarious closely related phenolic compounds which impart slight or noinhibition to popcorn polymer formation, for example, such closelyrelated compounds as:

2-diethylaminomethylphenol 2-diethylaminomethyl-4-t-amylphenol2-dimethylaminomethylphenol 2-diethylaminomethyl-4-nitrophenolZ-diethylaminomethyl-4-chloropheno12-dimethylaminomethyl-4-methyl-6-t-butylphenol2-dimethylaminomethyl-4-t-amylphenol 2-(N-ethyl-N-B-hydroxyethylaminomethyl) -4-t-amy1- phenol 2- (morpholinomethyl -4-t-amylphen ol2,6-bis(dimethylaminomethyl)-4-t-amylphenol 2- di-n-butylaminome thyl-4-t-amylphenol 2-dimethylaminomethyl-p-cresol Other valuable advantagesof the inhibitors of this invention are their chemical stability andtheir moderate propensity to cause discoloration of the syntheticrubber. Undue discoloration of the rubber products has been a seriousdrawback of many of the previously used inhibitors.

I claim:

1. The method of inhibiting the formation of popcorn polymer inprocesses for the preparation of synthetic rubber from olefinic monomerswhich comprises contacting the monomers with an inhibiting amount of atertiary amino naphthol compound having the structure wherein R and Rare selected from the group consisting of alkyl radicals having one tofive carbon atoms, 5- hydroxyalkyl radicals having one to five carbonatoms, and oxydiethylene when R and R together form a single radicals;and n is 1 to 2.

2. The method of claim 1 wherein the inhibitor is 1dimethylaminomethyl-Z-naphthol.

3. The method of claim 1 wherein the inhibitor is 1-diethylaminomethyl-Z-naphthol.

4. The method of claim 1 wherein the inhibitor is1-di-n-butyla|minomethyl-Z-naphthol.

5. The method of claim 1 wherein the inhibitor is 1-di(B-hydroxyethylaminomethyl)-2-naphthol.

6. The method of claim 1 wherein the inhibitor is 1-N-ethyl-N-fl-hydroxyethylaminomethyl-Z-naphthol.

References Cited UNITED STATES PATENTS 2,326,440 4/1941 Craig 260-68152,510,847 6/1950 Wilson 20257 2,326,440 8/1943 Craig 260681.5

OTHER REFERENCES I. H. Buckhalter et al.: I. Am. Chem. Soc. 68, p. 1894(1946).

H. R. Snyder et al.: J. Am. Chem. Soc. 70, p. 4230 (1948).

R. L. Shriner et al.: J. Am. Chem. Soc. 68, p. 946

DELBERT E. GANTZ, Primary Examiner J. M. NELSON, Assistant Examiner US.Cl. X.R.

